Why can't I send PMs.

In have successfully posted new topics, and replied to other threads. Today I tried sending a PM to another member, but wasn't able to do this. Every time I tried to send the message, a red box came up that said my message needed to be at least three characters long, and my message completely disappeared.- my message was definitely longer than three characters- I had given it a proper title in the title box- I selected the box to save my messages in my send box, but having failed to send, there was no copy of the message in my sent or draft boxes.Help appreciated!